On Mon, Mar 20, 2017 at 10:26:43AM +0100, Dmitry Vyukov wrote:
> On Mon, Mar 20, 2017 at 10:21 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Mon, Mar 20, 2017 at 3:28 AM, Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
> >> Hi Greg,
> >>
> >> Today's linux-next merge of the tty tree got a conflict in:
> >>
> >> drivers/tty/tty_ldisc.c
> >>
> >> between commit:
> >>
> >> 5362544bebe8 ("tty: don't panic on OOM in tty_set_ldisc()")
> >>
> >> from the tty.current tree and commit:
> >>
> >> 71472fa9c52b ("tty: Fix ldisc crash on reopened tty")
> >>
> >> from the tty tree.
> >>
> >> I fixed it up (see below) and can carry the fix as necessary. This
> >> is now fixed as far as linux-next is concerned, but any non trivial
> >> conflicts should be mentioned to your upstream maintainer when your tree
> >> is submitted for merging. You may also want to consider cooperating
> >> with the maintainer of the conflicting tree to minimise any particularly
> >> complex conflicts.
> >>
> >> --
> >> Cheers,
> >> Stephen Rothwell
> >>
> >> diff --cc drivers/tty/tty_ldisc.c
> >> index b0500a0a87b8,4ee7742dced3..000000000000
> >> --- a/drivers/tty/tty_ldisc.c
> >> +++ b/drivers/tty/tty_ldisc.c
> >> @@@ -621,14 -669,17 +621,15 @@@ int tty_ldisc_reinit(struct tty_struct
> >> tty_ldisc_put(tty->ldisc);
> >> }
> >>
> >> - /* switch the line discipline */
> >> - tty->ldisc = ld;
> >> tty_set_termios_ldisc(tty, disc);
> >> - retval = tty_ldisc_open(tty, tty->ldisc);
> >> + retval = tty_ldisc_open(tty, ld);
> >> if (retval) {
> >> - tty_ldisc_put(tty->ldisc);
> >> - tty->ldisc = NULL;
> >> - if (!WARN_ON(disc == N_TTY)) {
> >> - tty_ldisc_put(ld);
> >> - ld = NULL;
> >> - }
> >> ++ tty_ldisc_put(ld);
> >> ++ ld = NULL;
> >> }
> >> +
> >> + /* switch the line discipline */
> >> + smp_store_release(&tty->ldisc, ld);
> >> return retval;
> >> }
> >>
> >
> >
> > Peter,
> >
> > Looking at your patch "tty: Fix ldisc crash on reopened tty", I think
> > there is a missed barrier in tty_ldisc_ref. A single barrier does not
> > have any effect, they always need to be in pairs. So I think we also
> > need at least:
> >
> > @@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
> > struct tty_ldisc *ld = NULL;
> >
> > if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
> > - ld = tty->ldisc;
> > + ld = READ_ONCE(tty->ldisc);
> > + read_barrier_depends();
> > if (!ld)
> > ldsem_up_read(&tty->ldisc_sem);
> > }
> >
> >
> > Or simply:
> >
> > @@ -295,7 +295,8 @@ struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
> > struct tty_ldisc *ld = NULL;
> >
> > if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
> > - ld = tty->ldisc;
> > + /* pairs with smp_store_release in tty_ldisc_reinit */
> > + ld = smp_load_acquire(&tty->ldisc);
> > if (!ld)
> > ldsem_up_read(&tty->ldisc_sem);
> > }
>
>
>
>
> I am also surprised that callers of tty_ldisc_reinit don't hold
> ldisc_sem. I thought that ldisc_sem is what's supposed to protect
> changes to ldisc. That would also auto fix the crash without any
> tricky barriers as flush_to_ldisc uses tty_ldisc_ref.
Ok, I'm reverting this patch. Michael and Peter, please rework it and
resubmit.
thanks,
greg k-h
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